1. Field of the Invention
The present invention relates generally to improvements in or relating to processes for the selective separation of plutonium and uranium from one another and from other metals.
Nowadays, the processes of jointly separating plutonium and uranium from other metals and the separation of plutonium from uranium are almost always carried out using aqueous solutions. There are presently primarily two methods of particular interest, with each being used in several variations: the solvent extraction method and the ion exchange method. While both methods can, in general, be employed--also on a commercial scale--both still exhibit undesireable disadvantages.
Relative long residence times of organic solvents, agents adapted to form complexes, or resins, in higher radiation density ranges, lead to radiolytical destruction and, thereby, to losses in capacity and detrimental gas formation.
In the case of solvent extraction methods, the penetration of UO.sub.2.sup.2+ and Pu.sup.4+ through the phase boundary between water and organic solvent, as well as the concentration of precipitates at the phase boundary or interface, present kinetic problems.
When using the mixer-settler technique during solvent extraction, the stationary processing is rendered more difficult by density fluctuations in the organic and aqueous phases during successive extraction and re-extraction steps.
The reaction of ion exchange resins and complexformers, e.g. tributylphosphate, with concentrated HNO.sub.3 presents an unnecessary danger potential.
The separation accuracy of solvent extraction processes, as well as ion exchange processes during the separation of uranium from plutonium, and in the separation of the two elements from one another, is of such a low extent that the separation is generally carried out in several successive steps.
These difficulties are already apparent in the case of relatively well defined solutions obtained, for example, by the dissolution of nuclear fuel elements in concentrated HNO.sub.3. Additional difficulties are encountered in such processing when nonuniform solutions are treated, which possibly also contain difficult to define organic materials of fluctuating composition. For such waste waters there have not been advanced fully satisfactory treatment processes so that also the recovery of nuclear fuel elements from such waste waters, the transport, and the storage of remaining residues have not been solved satisfactorily.
2. Description of the Prior Art
A flotation process for inorganic ions has been described in DE-AS 1 166 113 (DE-AS=German patent publication) in which process in conformity with the loading of the ion to be floated, there is added to the ion-containing solution an anionic or cationic collector and in which the so-formed insoluble reaction product is floated or slurried under introduction of a gas and removed as a foam or froth. In accordance with this prior art process the uranium may be separated from uranylsulfate solutions, albeit with poor yields.
In German Pat. No. 2 817 029 there is disclosed a process for selectively separating and recovering uranium from accompanying metals. In accordance with this prior art process hydrochloric acid is added to an aqueous solution containing uranium until the UO.sub.2.sup.2+ forms anionic chloro-complexes, a surface active agent is added, and the resultant precipitate is floated in a flotation cell. Uranium is recovered from the floated precipitate. While this prior art process allows recovery of uranium at a higher yield in comparison to the process of German patent publication No. 1 166 113, its particular drawbacks reside therein that it is based on the use of hydrochloric acid and, accordingly, can not be utilized on a larger scale without precautionary measures due to corrosion.
In accordance with German Pat. No. 2 902 516, the process of German Pat. No. 2 817 029 is extended to solutions containing sulfuric acid. This provides yields and concentrations of uranium which are superior in comparison with the process of German patent publication No. 1 166 113. However, the flotation time is longer and the concentration or enriching is lower than suggested by the process according to German Pat. No. 2 817 029; on the other hand, the corrosion dangers have been reduced.
In any event, the foregoing references do not provide a teaching of how to separate plutonium from uranium by means of particular organic compound-assisted precipitation.